This application is based on and incorporates herein by reference Japanese Patent Application No. 11-300208 filed Oct. 21, 1999.
The present invention relates to a spark plug that is inserted into a combustion chamber of an engine, and suitable for use in an engine that has a thick engine head for a higher engine output.
In conventional spark plugs, as shown in FIG. 3, a cylindrical insulator 301 surrounding a center electrode 305 is held inside a metallic fitting 303 in such a manner that one end 302 of the insulator 301 protrudes from one end 304 of the fitting 303. A ground electrode 307 is fixed to the fitting 303 so that the ground electrode 307 faces the top end of the center electrode 305 protruding from the insulator 301 through a discharge gap 308. This spark plug is threaded into a thread hole 310 formed in an engine head 309 that defines a combustion chamber 320 therein.
The fitting 303 is formed, on its outer peripheral surface, with a thread part 311 and a taper part 312 from the side of the end 304. The thread part 311 is engaged with the thread hole 310 by turning the plug. The taper part 312 has a diameter gradually decreasing toward the thread part 311. The taper part 312 contacts a taper surface 313 formed on the thread hole 310 to restrict leakage of gas from the combustion chamber 320.
The fitting 303 is produced by a cold-forging and then machine-cut to form the taper part 312 and the thread part 311 in shape. The machine-cutting tends to produce traces of a cutting tool (tool mark) on the taper part surface, resulting in a high surface roughness. Further, the machine-cutting tends to produce deflection of longitudinal axes between the thread part 311 and the taper part 312. As a result, sealing characteristics of the taper part 312 is lessened.
In high output-type engines, the engine head is made thicker to ensure more coolant flow for higher cooling efficiency. The fitting of the spark plug for such engines generally has a reach length RL of more than 12 mm. As known well in the art, the reach length is defined as a length from the end 304 of the fitting 303 to the point where the diameter of the taper part 312 is 14.8 mm. In the case of a spark plug having a longer reach length for high output type engines, it must have a sufficient sealing ability because the pressure in the combustion chamber 320 increases.
It is therefore an object of the present invention to provide a spark plug that can provide a sufficient sealing ability for use in high output-type engines.
According to the present invention, a spark plug comprises a center electrode, a ground electrode and a tubular fitting for engagement with an engine head. The fitting has a thread part and a taper part from one end thereof toward another end thereof. The fitting is formed by cold-forging a low carbon steel so that the taper part has its cold-forged surface roughness of less than about 10 xcexcm. The fitting is machine-cut to provide the thread part which deflects less than about 0.15 mm from the taper part with respect to longitudinal axes. This fitting is suitably used for spark plugs for high output engines in which a reach length of the fitting is at least 12 mm.